1. Field of the Invention
The present invention relates to a control device for use in an injection molding machine, which molds molten resin by injecting from a distal end of a barrel. Synthetic resin material is heated and melted in the barrel.
2. Description of the Related Art
An injection molding machine has a cylindrical barrel and a hopper for inputting synthetic resin material.
The barrel has an injection nozzle at its forward end. The barrel contains an injection screw rotatable and movable forward and backward, and has heaters on its outer circumference.
The hopper is placed at the rear of the barrel, and feeds the input synthetic resin material into the barrel.
The barrel melts synthetic resin material by heating the material. The barrel pushes the heated molten resin toward the forward end by the rotation of the injection screw, and injects the resin from the injection nozzle to the mold.
The mold is kept closed by a mold open/close mechanism, and the molten resin injected from the injection nozzle fills the mold cavity.
Then, the injection nozzle is separated from the mold, and the mold is released from the mold open/close mechanism. A molded product is taken out from the mold.
A control device for the injection molding machine controls the temperature of the barrel which heats, melts and injects synthetic resin material.
Namely, the control device controls the temperatures of the rear part, middle part, front part and nozzle of the barrel to a predetermined temperature.
The control device sets the temperature of the rear part to the lowest temperature, sets the temperature of the middle part to about 10° C. higher than the temperature of the rear part, sets the temperature of the front part to about 20° C. higher than that of the rear part, and sets the temperature of the nozzle to about 10° C. higher than that of the rear part.
The control device also controls the temperature of the barrel when temporarily stopping the injection molding machine.
Namely, when the injection molding machine is temporarily stopped, the control device sets a shift minus temperature to uniformly lower the temperatures of the rear part, middle part, front part and nozzle of the barrel. The control device uniformly lowers the control temperatures of these parts by using the shift minus temperature.
For example, if the temperature of the rear part at molding is 300° C., the control device sets the temperature of the middle part to 310° C., sets the temperature of the front part to 320° C., and sets the temperature of the nozzle to 310° C.
When the injection molding machine is temporarily stopped, and if, for example, the shift minus temperature is set to −100° C., the control device lowers the temperature of the rear part to 200° C., lowers the temperature of the middle part to 210° C., lowers the temperature of the front part to 220° C., and lowers the temperature of the nozzle to 210° C.
When the operation is temporarily stopped in an injection molding machine, the flow of molten resin in the barrel is stopped. Thus, even if the control temperature of each part is lowered by using a shift minus temperature, a scorching caused by deteriorated resin occurs in some types of resins. Particularly, a scorching may frequently occur in the front part of the barrel where the control temperature is higher.
Therefore, the shift minus temperature can be set on the basis of the control temperature of the front part lowered to a value not to cause, scorching.
However, if the temperature of the front part is lowered, the temperatures of the rear part, middle part and nozzle of the barrel are unnecessarily lowered. As a result, a longer period of time is required to increase the temperatures of these parts of the barrel to a temperature value sufficient for molding when the injection molding machine is restarted. Furthermore, the molten resin in the barrel may deteriorate if the temperature is unnecessarily lowered.